git-p4: don't use name-rev to get current branch

git-p4: don't use name-rev to get current branch

git-p4 was using "git name-rev" to find out the current branch.

That is not safe, since if multiple branches or tags point at
the same revision, the result obtained might not be what is
expected.

Instead use "git symbolic-ref".

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>